The present invention relates to a method of, and a demodulator for, demodulating a double sideband amplitude modulated (A.M.) signal in a quasi-synchronous (Q.S.) area coverage scheme utilizing sideband diversity.
A Q.S. area coverage scheme is a technique extending the coverage area in mobile radio schemes by simultaneous operation of a number of amplitude modulated transmitters, with overlapping service areas and closely spaced carrier frequencies (within a few Hertz of each other). Such Q.S. area coverage schemes have been used in the United Kingdom for speech communication by major users, such as the police.
Quasi-synchronous operation not only extends the coverage area but intensifies the coverage by overcoming shadowing by terrain features and large buildings. In equal signal strength areas, however, the performance may be degraded by interaction between the several received signals. In areas with no multipath fading, a slow beat occurs between the several received signals and when the resultant signal nulls below the receiver threshold, there is a consequent loss of audio signal. In multipath fading areas, the fading of the individual transmissions will be uncorrelated by virtue of the geographic spacing of the transmitters. However the interaction between the transmissions causes the resultant signal received at the mobile radio to exhibit similar fading characteristics.
The dubious performance of Q.S. schemes in signal overlap areas, that is areas where signals from two or more transmitters overlap, is tolerable with speech transmissions because the redundancy of speech ensures that there is rarely any loss of intelligibility. However, there is a growing demand for medium speed data transmission between the base station and mobiles and interaction between transmissions in Q.S. area coverage schemes can be a major source of errors. Sideband Diversity, described in greater detail in two published articles of which one is entitled "Sideband Diversity: a new application of diversity particularly suited to land mobile radio" published in The Radio and Electronic Engineer, Vol. 48, No. 3, pages 133-139, March 1978 by Professor W. Gosling, J. D. Martin, R. J. Holbeche and G. Allen, and the other of which is entitled "An evaluation of a sideband diversity technique for data transmission on the forward path in a mobile radio area coverage scheme" published in the Radio and Electronic Engineer, Vol. 49, No. 10, pages 521 to 529, October 1979 by G. Allen, R. J. Holbeche and Professor W. Gosling, is a technique that utilizes the redundancy of A.M. signals to overcome the interaction between transmissions in Q.S. schemes and in so doing allows the diversity advantage offered by geographically spaced transmitters to be realized in multipath fading environments.
In a Sideband Diversity scheme, a constant phase shift over the audio frequency band is introduced between the modulation applied to the transmitters by wide-band phase shift networks. In a two transmitter scheme this phase angle would be 90.degree. and the resultant signal (Vr) received by a mobile is described by: ##EQU1## where v=Common received signal amplitude
.omega..sub.c =Carrier angular frequency PA1 m=Modulation index PA1 .omega..sub.m =Modulation angular frequency, and PA1 .delta..omega..sub.c =Frequency offset between the transmitters.
The factor in the square brackets in each term of this expression represents the slow modulation caused by receiving two signals with a small frequency offset that is .+-..delta..omega..sub.c t/2. However the modulation is no longer identical for the carrier and/or the two sidebands and when one sideband is nulled to zero the other one is at a maximum. Thus the information content of the transmission is no longer periodically destroyed as in conventional Q.S. schemes.
When a vehicle is in motion, the Doppler shift introduced can reduce the offset frequency to zero or increase it to a maximum value of (.delta..omega..sub.c +2.omega..sub.d) where .omega..sub.d is the Doppler shift, depending upon the direction of motion of the vehicle between the transmitters. Equation (1) can thus be re-written: ##EQU2## Where .phi., the radio frequency phase angle, can take on any value between 0.degree. and 360.degree. and may be stationary or not depending upon the offset frequency.
Conventional demodulators cannot be used to demodulate transmissions in sideband diversity operation because, in the case of receiving two equal transmissions, the spectra of the resultant signal changes from a conventional A.M. signal at a radio frequency phase angle .phi.=0.degree. to single sideband at .phi.=90.degree., to a double sideband suppressed carrier at .phi.=180.degree. and to single sideband at .phi.=270.degree.. Consequently a demodulator must be capable of coping with these variations in input signal. In order to effect coherent demodulation, it is necessary to provide a reference signal which conveniently can comprise the carrier or can be obtained from the double sideband signal when there is no carrier.
However a carrier locking loop will periodically lose its reference, and hence its lock, when the resultant carrier nulls to zero. Similarly, a system which derives the carrier information from the sidebands will also lose lock when one of the sidebands is zero. This will be illustrated with reference to FIGS. 1 and 2 of the accompanying drawings which show two known types of demodulator.
The block schematic circuit shown in FIG. 1 is known as the 2F, or squaring, loop and comprises an input terminal 10 to which a sideband diversity signal is supplied. This signal is squared in a squaring circuit 12 and the output is filtered in a bandpass filter 14 and applied to one input of a mixer 16. The mixer 16 forms a part of a phase lock loop 18. The loop 18 includes a local oscillator 20 whose frequency is adjustable in response to an error voltage. The output of the oscillator 20 is multiplied by two in a multiplier 22 and applied as a second input to the mixer 16. The output of the mixer 16 is applied to a low-pass filter 24 which produces a voltage which is used for adjusting the frequency of the oscillator 20.
In operation with a sideband diversity input signal as described by Equation 2, the signal obtained from the carrier after squaring and bandpass filtering is: EQU (1+cos .phi.(t)) cos 2.omega..sub.c t (3),
the carrier signal derived from the sidebands is: EQU (0.5 cos .phi.(t)).multidot.cos 2.omega..sub.c t (4),
and the composite carrier signal is therefore: EQU (1+1.5 cos.phi.(t)).multidot.cos 2.omega..sub.c t (5).
This composite signal no longer has a single null at .phi.=180.degree. but two nulls occuring at .phi.=132.degree. and .phi.=278.degree.. These phase lock loop will lose lock at these phase angles and is therefore not suitable for sideband diversity operation.
The block schematic circuit shown in FIG. 2 is known as a Costas loop and comprises an input terminal 10 to which the sideband diversity input signal is applied. The input signal is applied to a first input of respective first and second mixers 26, 28. A local oscillator 30, in the form of a voltage controlled oscillator, is connected to the second input of the first mixer 26 and, via a 90.degree. phase shifter 32, to the second input of the mixer 28. The outputs of the mixers 26, 28 are applied to respective bandpass filters 34, 36 which pass the sideband signal components from the respective mixers. These sideband components are mixed in a further mixer 38 to produce an error signal E.sub.s. This error signal E.sub.s is filtered in a low-pass filter 40 to provide a voltage for adjusting the frequency of the local oscillator 30 as desired.
With a sideband diversity input signal as described by Equation 2, the Costas loop produces an error signal E.sub.s from the sidebands described by: EQU E.sub.s =m.sup.2 V.sup.2 /8.multidot.cos .phi..multidot.sin 2.theta.(6)
where .theta. is the phase error in the local oscillator.
If desired, an error signal can be obtained from the carrier by replacing the bandpass filters 34, 36 in the loop arms by low-pass filters. The composite signal so derived suffers from the same problems as those described for the 2F loop and the loop will lose lock at specific values of .phi..